1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus such as a printer or a copying machine.
2. Description of Prior Art
Area gradation method is used for many image forming apparatuses for expressing gradation with coarse or dense distribution of dots or lines. Many images formed by such an image forming apparatus such as a copying machine has various sections including a uniform density areas or half-tone density areas therein. Therefore, it is required to form an image with reproducible colors and densities. After an electrostatic latent image is formed on a photoconductor, toners are adhered onto the latent image to form a toner image. However, as the amount of charges on toners are changed with humidity or temperature, the extension (two-dimensional change in lateral directions) or the thickness (three-dimensional change in height direction) of toners adhered to an electrostatic latent image are changed. Then, it is desirable to stabilize an image by detecting the change in extension and height of toners adhered onto the photoconductor precisely to control the image forming conditions such as development bias voltage.
If the output of the sensor used for the image stabilization scatters, changes in extension and in height of toners adhered onto the photoconductor cannot be detected sufficiently precisely in order to control the image forming conditions. Therefore, it is desirable to increase the accuracy of the sensor output.
Further, in an image forming apparatus using area gradation method, when gradation characteristic is changed, the dot size of the dots or lines formed on the photoconductor and the amount of toners adhered to a unit area have also to be controlled. However, if the development bias voltage is shifted in order to change the amount of adhered toners, the dot size is also changed. Further, if the surface potential of the photoconductor is shifted in order to change the dot size, the transfer efficiency is also changed. Then, the image forming becomes unstable when gradation characteristic is changed. Then, it is desirable to control the image forming conditions so as to stabilize image formed with desired gradation characteristics.